Typical video display systems use analog techniques to create a vertical deflection signal. In a cathode ray tube (CRT) display, the CRT has a phosphor screen located opposite an electron gun. The electron gun emits electrons, which are accelerated and focused on the phosphor screen by a high voltage grid. The phosphor has a luminance that will only last for a short time interval, and hence the screen must be periodically refreshed to retain an image. Conventional monitors typically refresh the phosphor screen at 60 Hz.
An image is displayed on a phosphor screen using a raster scan technique. The screen is divided up into a number of horizontal scan-lines. At the beginning of displaying a new image, the electron gun is positioned at the upper left corner of the screen at the first scan-line. For each scan-line, the electron beam is steered horizontally (left to right) across the screen at a fixed frequency. When the electron gun reached the right edge of the screen, the electron gun must return back to the left side of the screen (termed the horizontal retrace). During horizontal retrace, the electron beam is steered (right to left) to the left edge of the next scan-line immediately beneath the preceding scan-line. Once all scan-lines are traced by the electron gun, the beam is steered back to the top left corner of the screen during the vertical retrace interval. As described above, a CRT with a raster scan display screen requires the electron beam to be steered horizontally across the screen, and vertically across the screen for both the horizontal retrace interval and the vertical retrace interval. A horizontal deflection circuit is used to steer the beam horizontally, while a vertical deflection circuit steers the beam vertically. The horizontal and vertical deflection circuits produce high voltage signals. The high voltage signals are used to activate deflection coils, which steer the electron beam horizontally and vertically.
Typical vertical deflection circuits include a vertical oscillator circuit and vertical deflection coils. A composite video signal is coupled into the vertical deflection circuits. The composite video signal has horizontal and vertical sync pulses embedded in the signal. The vertical sync pulses in the composite signal are used to trigger the vertical oscillator so that the vertical oscillator locks to a frequency of 60 Hz. The vertical oscillator generates a 60 Hz saw tooth waveform. The saw tooth waveform is used to generate a current ramp to drive the vertical deflection coils. The current ramp drives the vertical deflection coils such that the electron beam moves from the top of the screen to the bottom of the screen at a uniform rate. At the end of the current ramp, the deflection coils are deactivated and the electron beam returns to the top of the screen.
Other circuits within a CRT display using vertical rate waveforms may use parabolic shape waveforms. Such circuits correct for parabolic errors that arise due to the geometry of the CRT, distortions in the deflection coils, and the variation in focus voltage between the center and the top and bottom of the screen. A parabolic waveform may be used to modulate the amplitude of the horizontal rate scan, in order that vertical lines portrayed on the screen appear as straight. Similarly, the focus voltage applied to the CRT focus electrode often needs to be modulated with a parabolic waveform in order to adjust the focal point of the electron beam as the beam scans from top through center to the bottom of the screen.